Next I put in the main shape for the tree and start refining and correcting the main shapes of the building. The brush I used for the tree is a round brush with "shape dynamics" set to pen pressure for size and opacity at 100%. This way I can get some very hard edges.

It is a good idea to regularly flip your image so that you can see it from a new perspective. This will allow you to see errors more clearly, as well as evaluate your composition. Usually I flip my image every few minutes. Don't just flip your image and then flip it back, but flip it, paint on the flipped version, flip it back, paint on that version, and so on. I am still always correcting my shapes.